In the field of battery powered electronic devices such as mobile communication devices and the like, power management is an important feature and is often provided by way of a dedicated power management integrated circuit (PMIC) device. A key component of such a PMIC device is battery state-of-charge functionality, which tracks current entering and leaving the battery (e.g. by way of a Coulomb counter), and based on this information is able to estimate the stage-of-charge for the battery. Such state-of-charge functionality allows, for example, the remaining talk time and standby time in hours and minutes of a mobile communication device to be predicted.
The Coulomb counter is used to integrate the current drawn from or injected into the battery to yield the status of the battery charge. The output of the Coulomb counter may be processed by software together with, say, battery age, temperature, battery voltage, indication of full re-charge by charger, and a number of charge/discharge cycles to estimate the remaining capacity of the battery. The Coulomb counter is required to have a very low current consumption due to it having to always be ‘on’, which restricts the sampling and operating frequency of the converter. Nevertheless, the Coulomb counter component is also required to capture fast input transients.
FIG. 1 illustrates an example of a conventional implementation for a Coulomb counter 100 used as part of a battery state-of-charge functionality within a PCIM device 105. The voltage (Vsns_diff) across a sense resistor 110 is filtered using a single pole RC filter, which provides a form of ‘memory’ that enables the fast input transients to be captured whilst enabling a sampling and operating frequency of the Coulomb counter to be kept low. In the illustrated example, the RC filter is implemented by way of an external capacitor 120 and internal resistances 130, 135. The filtered voltage is then provided to, for example, a first order incremental converter 140 using, for example, switched capacitor technology (not shown) clocked with the RTC (real time clock) of the electronic device (e.g. 32 KHz).
A high resolution is required for the converter 140, which is achieved by a large oversampling ratio:
                                                        conversions_cycle              ⁢              _frequency                                      input_signal              ⁢              _bandwidth                                ≈                      10            ,            000                          ⇒                              LPF_cutt            -            off_frequency                    ≈                      1.6            ⁢                                                  ⁢            Hz                                              [                  Equation          ⁢                                          ⁢          1                ]            
The use of an external capacitor 120 means that its size is restricted by PCB (printed circuit board) constraints. Accordingly, in order for the RC filter to achieve the required input signal bandwidth, the internal resistances 130, 135 have to be sufficiently large to compensate for the limited size of capacitance (f=1/(2.pi.R.C)); e.g. R=2×500K, C=100 nF.
For the next generation of devices, external components (i.e. the capacitance 120) are required to be much reduced or removed, but the converter bandwidth cannot be increased too much due to current consumption constraints. Consequently, the integrated resistors 130, 135 will be required to be significantly larger to compensate for the reduction or removal of the external capacitance 120, while keeping the cut-off frequency for the input low pass filter approximately the same, which is necessary to avoid increasing the converter bandwidth. However, the required high value input resistances result in the filter introducing a differential input voltage error as large as 100 times the required resolution. Such a large differential input voltage error introduces a prohibitively large gain error into the Coulomb counter.